The present invention relates generally to the field of electronic weighing scales, and more particularly to electronic scales designed particularly for use in weighing of mail pieces to determine the amount of postage required for mailing.
Postal weighing scales, i.e., scales designed particularly for use in weighing mail, have long been well known, and many varieties of such scales have been developed, both for general use such as in industry mail rooms and offices, and by the US Postal Service in post offices and mail distribution centers. These scales typically range in capacity from one pound to as much as 200 pounds, depending on whether a particular scale is used primarily for letter mail or for heavier mail or packages. Earlier versions of these scales utilized various forms of mechanical balancing devices so that a load placed on the scale platter would cause a mechanically operated weight indicator to register the weight of the load on a visible scale, or cause a balance arm to seek a midpoint when a load equal to the weight of the load on the platter was applied to the arm. These scales were highly successful in operation, for the most part, and met with a considerable degree of commercial success.
However, as in so many other forms of mechanical devices, the advantages of converting to electronic operating components became apparent. A principal factor involved in the conversion of postal scales from mechanical to electronic operating components was the fact that postal scales must have an extremely high degree of accuracy under virtually all operating conditions, and scales having electronic operating components were found to be consistently more accurate than those having mechanical operating component. The reason for the need for such high degree of accuracy is that postal scales are used for determining the amount of postage that must be applied to letter and small package mail, and to larger packages being mailed as parcel post, and even very slight inaccuracies in the weight given by a scale could potentially cause a serious loss of money, either for the mailer or for the USPS, depending on the manner in which the scale was inaccurate. This can be better appreciated when one considers the volume of the different types of mail handled by the Postal Service each year, which typically is in the billions of pieces.
One problem that arose with scales having electronic operating components is that the load cell, which is the heart of an electronic scale, is a highly delicate instrument and is subject to damage in the event that it is subjected to a load in excess of the load for which it is designed. Basically, a load cell using strain gage technology can be a generally rectangular metallic body member which is adapted to have one end rigidly mounted on a frame so that the load cell is supported only at that end, with the rest of the body member being cantilevered from the mounting end. The other end of the body member is provided with some means for supporting a weight. Strain gages are mounted on the body member in appropriate locations that very slightly when the body member is deflected by the application of the weight. An electric voltage is applied across the strain gage which varies in accordance with the extent to which the strip is strained by the weight on the free end of the body member. Generally, there are several strain gages located at appropriate locations that allow them to be connected in the form of a Wheatstone Bridge, which is well known in the art. By suitable electronic devices also well known in the art, variations in the voltage across the bridge circuit metallic strip resulting from different weights applied to the free end of the body member can be read as different weights on various forms of digital readout devices.
One of the major advantages of the load cell as a measuring device for use in postal scales is that it is extremely accurate, permitting scale readings to within one part in 3000 to 5000 on a five pound scale. There is, however, a tradeoff in that the accuracy of the load cell is dependent upon a very sensitive construction that is easily damaged with overload. The problem is that with a typical load cell, the deflection of the free end of the cell is extremely small, in the order of ten thousandths of an inch, and if the load cell is deflected much beyond that amount, the bond of the strain gage to the base metal or the base material that stretches in response to deflection of the body member is permanently damaged, rendering the load cell useless. Thus, it is very important to provide a scale having a load cell control component with suitable means for preventing the load cell from being subjected to an excessive load, such as would occur if a user placed a load on the scale platter which exceeded the maximum weight for which the scale is rated, or possibly mishandled the scale in such a manner that a momentary impact on the scale platter caused the overload.
In the prior electronic postal scales over which the scale of this invention represents an improvement, as represented by the scale shown in U.S. Pat. No. 5,072,799, issued on Dec. 17, 1991 to Freeman et al., and assigned to the assignee of this application, the load cell is mounted between a pair of identical cast metal body members, each of which has a central portion to which opposite ends of the load cell are connected. Each of the body members also has a plurality or legs extending radially outwardly, those from the lower body member supporting the assembly of the body members and the load cell in a suitable frame or housing, and those from the upper body member providing a suitable weight distribution plate for supporting the scale platter, thereby supporting the scale platter adjacent the four corners thereof. The load cell is protected against overload by an adjustable center downstop which includes a pair of downstop set screws which seat against a metal surface, such as the head of a bolt, and a controlled gap at the corners due to machining portions of the castings at each of the corners.
Because of the particular use of the scale, i.e., to indicate the amount of postage required for mailing, the adjustability of the downstop had to be very critical, typically being in the range of 8 to 30 thousandths of an inch, with an adjustment tolerance of plus or minus one thousandth. As previously indicated, the actual deflection of the free end of a load cell is very small, and it was found, in the case of the cast metal body members of the previous scales, that the machining tolerances on the castings would not permit variations in the position of a load cell downstop to plus or minus one thousandths of an inch, particularly when there are two different components that are involved in the tolerance buildup. The height of the load cell, plus the tolerance of the machining of both surfaces of the downstop and the load cell mount on each of the two cast body members all contribute to rendering such a small variation in adjustability of the downstop virtually impossible without an adjustable downstop. Thus, the adjustable center downstop was required. Adjustable corner downstops were not required because the gap required between the ends of the legs of the case body members was much larger at the corners than was the deflection of the free end of the load cell, in the order of 100 to 125 thousandths of an inch, plus or minus 5 thousandths. Therefore, these portions of the cast body members could be machined to produce the required tolerance variations.
It should be understood that, while case metal body members with carefully machined parts and very accurate adjustable center downstops are well suited to the manufacture of a substantially large scale, such as would be used for weighing packages up to 200 pounds, this type of construction is too costly for smaller scales, such as five pound capacity scales that are used primarily to weigh letter mail, which renders them relatively noncompetitive. Thus, the need was recognized for an entirely different form of weight distribution plate and overload protection system for small capacity scales. The present invention accomplishes these objectives, as described in detail hereinafter, by eliminating the cast metal body members and also the adjustable center downstop, thereby greatly reducing the mechanical complexity of the scale, the cost of manufacturing, and the need for critical adjustment of movable parts.